The concept of zero is surprisingly difficult to grasp, even for people.

"There is some understanding of nonexistence that seems to develop naturally, but the actual use of the term 'zero' seems to need to be taught," said comparative psychologist Irene Pepperberg. Pepperberg conducted the study at Brandeis University in Waltham, Massachusetts.

The idea of zero as a nonexistent quantity was not obvious to early human cultures, Pepperberg said. Most Europeans lacked a term for it until the 1600s.

Researchers in the United States and Japan have previously shown that chimpanzees and possibly squirrel monkeys can comprehend zero when taught. Chimps have also used it when adding and subtracting quantities of objects. Now a precocious 28-year-old parrot named Alex may have provided the first evidence of animals other than primates getting it too.

Zero Sum

For 27 years Pepperberg has tutored Alex to understand the English names of 50 different objectssuch as "banana," "truck," and "grape"plus the names of seven colors, five shapes, and the numbers one through six.

Pepperberg's most recent research with Alex, co-authored with Brandeis graduate student Jesse D. Gordon, is detailed in the current issue of the Journal of Comparative Psychology.

It not only shows that Alex can count jelly beans, colored blocks, and other objects but also hints that he may have spontaneously come up with a "zero-like concept."

During one experiment Alex was presented with blocks in differently colored sets of two, three, and six. When researchers asked Alex which color group had five blocks, he answered, "None." This prompted Pepperberg to set up a series of tests in which the parrot consistently identified zero quantities of objects with the label "none."

Alex had been taught the term "none" to indicate when neither of two identically sized objects was larger than the other. He had also used it to indicate when there was no difference in other qualities, such as color or shape, among a set of objects.